Certain thiosemicarbazones--especially substituted isatine 3-thiosemicarbazones--have an antiviral activity. These have been reviewed by Bauer [Bauer, D. (ed). In: Chemotherapy of Viral Diseases: Thiosemicarbazones (1972) Vol. 1, Pergaman Press, Oxford (5) in U.K. Patent No. 975357, 1964 to Wellcome Foundation, which relates to N-methylisatine 3-thiosemicarbazone (M-IBT)--commercially known as Methisazone or Marborane), and which is one of the few clinically effective synthetic antiviral drugs acting by the inhibition of replication of DNA containing pox viruses. It has been established that N-methylisatin-.beta.-4',4'-diethylthiosemicarbazone (in the following M-IBDET) inhibits the production of Moloney leukemia virus (1-3). This does not imply, and does not permit any prediction, as regards the possible activity of this type of compounds against animal and human diseases caused by other retroviruses. As a matter of fact, the well-known Marborane has been tested by us and has been found to be ineffective against the viruses of the Retroviridae against which the compositions of the invention are effective.